What is the ruling on stipulating to the husband during divorce that the mother be granted custody of the child if the father remarries, knowing that this is according to the Hanbali school of thought?

There is no need to stipulate the mother's custody of the child after separation, for the mother has a greater right to her child's custody unless there is an impediment to custody agreed upon by the majority of jurists. If the intent is to stipulate custody for the mother in a situation where custody belongs to the father, then the matter is subject to scholarly disagreement among jurists as to whether the right of custody belongs to the custodian or to the child, and the Shariah judge will decide on this.